1. Field of the Invention
The present invention relates to a semiconductor device and a method for driving the semiconductor device, and in particular to a semiconductor device including a plurality of analog-to-digital (A/D) converter circuits and a method for driving the semiconductor device. Further, the present invention relates to a semiconductor device in which the A/D converter circuits are used as output circuits and a method for driving the semiconductor device.
2. Description of the Related Art
With the progress of miniaturization of semiconductor elements, a larger capacity of memories and higher performance of CPUs have become possible. As a new function added to digital electronic devices including such memories or CPUs, sensor technology has attracted attention. For example, by mounting acceleration sensors or gyro sensors on portable media players, smartphones, or the like, a range of use thereof can be expanded dramatically. Further, as image pickup elements of various kinds of cameras or video cameras or as sensors of optical mice or two-dimensional barcode readers, image sensors that capture images are practically used in many fields.
Image sensors are devices capable of generating a subject image in such a manner that an optical sensor in each of pixels 601 that are arranged in a two-dimensional array as illustrated in FIG. 6 receives light emitted or reflected from a subject, the brightness of the received light is converted (photoelectrically converted) into electric signals, and the electric signals are sequentially read out from each pixel. Currently-used general image sensors are classified roughly into two types: a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor.
A CCD image sensor sequentially carries electrons accumulated by photoelectric conversion in pixels that are arranged in a two-dimensional array to an output circuit by its transfer function and outputs signals from the output circuit. A CMOS image sensor selects pixels, in which electrons are amplified, by X-Y addressing and outputs signals from an output circuit. In the CCD image sensor, all the pixels are addressed at the same time and signals are output only in the arrangement order of the pixels, while in the CMOS image sensor, switching of transistors are controlled and addressing is repeated on a row basis so that signals can be output regardless of the arrangement order of the pixels. As the output circuits of the CMOS image sensor, parallelly-provided A/D converter circuits are mainly employed. The parallelly-provided A/D converter circuits can speedily convert signals that are output from photosensors of the pixels.
In the case of incorporating the above-described sensors in electronic devices, A/D conversion is necessary. A/D conversion means converting analog signals into digital signals. A circuit which performs A/D conversion is called A/D converter circuit. For example, signals output from photosensors in pixels of an image sensor are analog signals, and in order to use the analog signals directly in an electronic device, the analog signals need to be converted into digital signals. To obtain desired digital signals, circuit design of a signal path is extremely crucial.
That is, it is desirable to properly design an A/D converter circuit that takes accuracy and resolution into consideration in accordance with the performance needed for desired digital signals. The accuracy of the A/D converter circuit is indicated by S/N ratio. S/N ratio is a ratio of signal to noise, and a higher S/N ratio means higher accuracy. In addition, a lower resolution causes a larger error at the time of converting analog signals into digital signals, thereby making it difficult to obtain a favorable S/N ratio.
As types of the A/D converter circuit, there are an integrating type, a successive approximation type, and a flash type, for example. In the integrating type, an integrator whose input voltage is a measurement voltage is used and an output value is obtained from a rate of change of an output of the integrator. Patent Document 1 discloses an integrating A/D converter circuit that uses low-frequency clocks for A/D conversion for the purpose of small power consumption and has a wide dynamic range. In the successive approximation type, a comparator compares a reference voltage obtained as an output of a D/A converter circuit with a measurement voltage; based on the comparison results, the output of the D/A converter circuit is sequentially changed; thus, an output value is obtained. Patent Document 2 discloses a successive approximation type A/D converter circuit whose processing time is effectively shortened by reducing the time required to re-execute comparator operation and to perform switching from the comparator function to the A/D conversion function. In the flash type, different reference voltages are input to a plurality of comparators, a measurement voltage is compared with the reference voltages, and an output value is obtained from the comparison results.
Further, with the recent rapid progress of miniaturization of pixels in the above-described sensors, size reduction of a peripheral circuit on a chip is also expected. CMOS image sensors have a great advantage of integrating various functional circuits on a chip; however, they tend to have a significant problem in size reduction of A/D converter circuits that are provided in parallel for columns of a pixel array. In order to obtain desired digital signals, the A/D converter circuits are required to have high performance as well as to have a reduced size.